In the case where it is required to provide some processing at a desired area of an image displayed on a screen of a display apparatus, it is common practice to display a rectangular frame, called a box cursor, for defining the area. It is convenient to display the box cursor by inverting specific bits of those representing the image. The reason is that, by inverting again the specific bits inverted for displaying the box cursor, the box cursor can be easily removed and the original image at the position where the box cursor is displayed can be restored.
Now, it is often required to display two box cursors. For example, two box cursors might be used where a portion of an image in one box cursor is moved to an area within another cursor, either as it is or by enlargement. Usually, two box cursors consist of sides or lines with same thickness, so that, if they overlap partially with each other, the overlapped portion is erased. That is, the bit inverting operation is performed twice for the overlapped portion, so that it becomes in a state where no bit inverting operation is performed. For example, as shown in FIG. 8, the overlapped portion 83 of two box cursors 81 and 82 is erased.
A prior art technique dealing with such problem of erasure is disclosed in pages 649-650 of IBM Technical Disclosure Bulletin, Vol. 27, No. 1B, June 1984. That technique is characterized in that, as shown in FIG. 9, the lines of two box cursors are arranged to have different thicknesses (widths). For example, one box cursor 91 is represented by lines with a thickness of one bit (pel), while the other box cursor 92 is represented by lines with a thickness of two bits. Thus, even if two box cursors overlap with each other as shown, and an overlapped portion is erased, a non-overlapped area 93 is left for the box cursor 92 consisting of thicker lines, so that the function for defining the region is maintained to some extent.
In the prior art technique which was two box cursors which differ in line thickness, the apparent size of the box cursors varies slightly at or near a location where they overlap as shown in FIG. 9. That is, a portion 94 of the thinner box cursor 91 overlaps the thicker box cursor 92 and is erased. Instead, a remaining portion 93 of the thicker box cursor is displayed as if it is a part of the thinner box cursor 91 to provide the impression that the latter is slightly reduced. However, if the overlapped area is near the inside of the box cursor 92, the box cursor 91 is displayed such that it is slightly enlarged. If the lines for both box cursors are relatively thin, the display does not provide so much of an unnatural impression. However, if they are relatively thick, the unnatural impression becomes significant.
Also, in the prior art technique, if the number of box cursors to be displayed simultaneously is more than two, it is required to display the third and fourth box cursors in lines that are to be successively thicker. These cursors are unshapely and not practical.